---
title: "#48 Rotate Image"
tags: LeetCode, Top100
---
#48 Rotate Image
==

題目描述
--
You are given an *n x n* 2D ==matrix== representing an image, rotate the image by 90 degrees (clockwise).

You have to rotate the image ==**in-place**==, which means you have to modify the input 2D matrix directly. **DO NOT** allocate another 2D matrix and do the rotation.

Example 1:
--
>Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
Output: [[7,4,1],[8,5,2],[9,6,3]]

解題思維
--
如果照著題目意思轉90度，很難完成in-place的要求。
我們仔細觀察矩陣位置，發現：如果我們先將其矩陣上下顛倒
>[[7,8,9],[4,5,6],[1,2,3]]

再對角線交換，也能達到90度旋轉。

Logic
--

class Solution:
    def rotate(self, matrix: List[List[int]]) -> None:
        """
        Do not return anything, modify matrix in-place instead.
        """
        matrix.reverse()
        n = len(matrix)
        for i in range(n):
            for j in range(i, n):
                if i == j:
                    continue
                matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]